1. Field of the Invention
The present invention relates to a light scattering apparatus and light scattering measurement method, and more particularly, to a light scattering apparatus and light scattering measurement method enabling measurement even when a sample to measure is opaque.
2. Description of Related Art
A dynamic light scattering method is to analyze a fine structure and motion characteristics inside a solution by measuring thermal motion occurring inside the solution through the light scattering phenomenon. Generally, measurement using light is difficult to measure a structure under wavelengths of the light in principle. However, the dynamic light scattering method is to measure an internal structure through thermal motion, and therefore, enables measurement of a fine structure under wavelengths of the light (from 0.1 nm up to and including 1 μm). Accordingly, the dynamic light scattering method using visible laser light source relatively safe to human bodies is a convenient observing method enabling determination of a size of a particle dispersed inside a solution and of a network structure of gel. The dynamic light scattering method has widely been used as a method to directly measure a particle size in a solution-state in a transparent particle dispersion system.
For example, as a practical application, it has been tried earlier applying the dynamic light scattering method to measurement of transparent samples with complicated structures such as liquid crystal, gel and colloid, but, no practical use has been known currently. This is because scattered light from liquid crystal or gel inevitably includes an excessive component caused by inhomogeneity of an internal structure, and there is a problem that no method exists to evaluate such a component accurately.
In recent years, in order to solve such a problem, a method has newly been developed to determine a measurement amount of static average (ensemble average) with time average and spatial average properly carried out in close consideration of an excessive component of scattered light caused by inhomogeneity using a scanning microscopic scattering method. See Document 1 (H. Furukawa, K. Horie, R. Nozaki and M. Okada “Swelling-induced modulation of static and dynamic fluctuations in polyacrylamide gels observed by scanning microscopic light scattering”, Phys. Rev. E, 68, 2003, p. 031406-1 to 031406-14), Document 2 (H. Furukawa and K. Horie “Characterization of a fine network structure of a polymer gel by scanning microscopic light scattering”, Japanese Journal of Polymer Science and Technology, Vol. 59, 2002, p. 578-589). Thus, the method has been established for measuring a transparent sample with a complicated fine structure using a visible light source. Further, Document 3 (H. Furukawa and S. Hirotsu, “Dynamic Light Scattering from Static and Dynamic Fluctuations in Inhomogeneous Media”, J. Phys. Soc. J., 71, 2001, p. 2873-2880) describes precisely deriving a mathematical formula necessary to determine specifically an ensemble average correlation function. In this way, practical methods have been established for directly measuring a fine internal structure of a transparent sample with a complicated internal structure using a visible light source.
However, the aforementioned methods are not effective as a method for measuring an internal state of an opaque gel substance or colloidal solution including food such as milk and pudding and cosmetics such as shampoo and conditioner. In the dynamic light scattering method, measurement of a fine internal structure is easy in transparent samples using visible light, but difficult in opaque samples (including samples with high turbidity). A phenomenon of being cloudy is a phenomenon where a substance seems to be opaque because strong scattering occurs and causes multiple scattering such that scattered light is scattered again. Accordingly, specific consideration is required to detect light that is scattered only once as in the general light scattering method.
Conventionally, as a method of overcoming such a problem, there have been a method of removing the contribution of multiple scattering using two laser beams, and a method of measuring in close proximity to a surface to ignore multiple scattering. See, respectively, Document 4 (L. B. Aberie, P. Hulstede, S. Wiegand, W. Schroer and W. Staude, “Effective suppression of multiply scattered light in static and dynamic scattering”, Applied Optics, 37, 1998, p. 6511-6524), and Document 5 (Otsuka Electronics Co., Ltd. “Fiber-Optics Particle Analyzer FPAR-1000”, online, Searched Aug. 1, 2004, [http://www.photal.co.jp/product/fpar—0.html]). However, it is known that the former is complicated and expensive and therefore not spread widely, and that the latter is commercially available, but cannot obtain accurate measurements because the means for overcoming the problem is too simplified.
Meanwhile, there is theoretical consideration already about what scattering occurring inside an opaque sample, suggesting that analyzing multiply scattered light enables in principle examination of thermal motion inside a sample.
However, it is necessary to know the number of scattering times N of multiple scattering occurring inside a sample in applying the theory to examine thermal motion inside the sample by analyzing multiply scattered light. Determining the number of scattering times N is difficult both theatrically and experimentally, and therefore, examining internal thermal motion is difficult in principle in a state where multiple scattering occurs. Further, the number of scattering times N is an average amount, and the accurate number of times cannot be determined. Accordingly, even if a method is established of assuming the number of scattering times N experimentally, problems arise such that an internal structure is only determined within some error range in existing theories even using such a method.
Further, in order to perform inexpensive practical analysis, it is required to promptly process time-series data of an enormous amount of photon pulses measured in dynamic light scattering. A specific analysis apparatus has widely been used in processing of time-series data for accurate measurement, is high in evaluation, but expensive. The analysis apparatus is a light scattering dedicated calculator in which all calculation portions are made of logic circuits (so-called full-logic), which are complicated circuits, resulting in limitations in reduction in manufacturing cost.